A varactor is a semiconductor device having a voltage-sensitive capacitance. Frequently, the space-charge region and the accumulation at the surface of a semiconductor contacting an insulator are altered as a function of applied voltage to produce a bias-dependent capacitance.
Varactors are used in semiconductor applications, for example, to construct voltage-controlled oscillators (VCO). Use of a VCO is a cost-effective method for generating a tunable stable frequency without employing a circuit with multiple oscillators. U.S. Pat. No. 7,129,801 to Wu shows an exemplary use of a varactor in a VCO circuit. A VCO is a versatile basic building block for constructing transceiver circuitry, phase locked loop (PLL) circuitry, and other wireless communication circuitry.
The tuning characteristic of a varactor can affect their circuit performance significantly. The Q factor of a VCO circuit depends on the ratio of the maximum capacitance to the minimum capacitance, that is, Cmax/Cmin. While the use of a modified trench capacitor, which is commonly used in dynamic random access memory (DRAM) devices as a varactor is known in the prior art, the ratio of a maximum capacitance to a minimum capacitance is relatively low for this type of varactors with a poor processing control to obtain intended tuning ratio.
High tunability of a VCO circuit requires a high Q factor, and consequently a high number for Cmax/Cmin. The varactor disclosed in U.S. patent application Ser. No. 11/617,322 supra requires formation of pillar structures, which necessitate additional processing steps in a semiconductor manufacturing sequence.
Therefore, there exists a need for a semiconductor varactor that provides a high number for the ratio of a maximum capacitance to a minimum capacitance with minimal number of additional processing steps in a standard semiconductor manufacturing sequence.
Furthermore, requirements for the value of Cmax/Cmin depend on the nature of the semiconductor circuit. Therefore, there exists a need for the capability of altering the value of Cmax/Cmin by adjusting process parameters, design parameters, or alterations in the semiconductor varactor structure.